Struct PanelCondition 

pub struct PanelCondition {
    pub panel: &'static str,
    pub when: fn(&ButtonState<'_>) -> bool,
}

A panel whose visibility ALSO depends on the document, beside the claim.

The claim system answers “which workbench is this panel’s”. It cannot answer “does this document have anything for it”, because a Workbench’s panels is a list of ids with no room for a predicate. This is that second half, in the shape WorkbenchButton::when already uses — a predicate over ButtonState, which carries the whole engine — so a conditional PANEL is declared exactly like a conditional BUTTON and read in exactly one place (panel_visible).

Keyed by PANEL rather than by workbench because the condition is the panel’s own: Qualify wants a part with connection points wherever it is drawn, not a different answer per workbench.

Fields

panel: &'static str

when: fn(&ButtonState<'_>) -> bool

Whether the document has anything for this panel. false hides it exactly as an unclaimed workbench does — in place, keeping its position.